This invention relates generally to the underwater guns, and more particularly to the line releasing devices for said guns, and it is its main object to provide a line releasing device for said guns which is very simple yet reliable in its operation. According to one aspect of the invention, the line releasing device includes a dog member, fulchrumed at one end around a vertical axis to the trigger guard of the gun, so as to rock in a horizontal plane, from a line retaining position to a line releasing position. The said dog member is provided with a sidewise outwardly projecting arm, to which the line may be releasably hooked. The central portion of said dog member is provided with a notch, which may be releasably engaged by the lower end of the trigger, so as to lock the dog into line retaining position, until the trigger is operated. In proximity of said notch, the dog member is cam shaped so that the trigger, whenever coming back to its rest position after firing, will automatically reset and lock the dog member into its line retaining position.